This study is designed to investigate in depth the structure, function, and thiogenic significance of a newly described human serum low density lipoprotein inhibitor (LDL-In) which may represent a normal physiological repressor of lymphocyte action. LDL-In will be purified, biochemically characterized, and the biologically active moiety identified. The cellular sites of action, and the pathway by which LDL-In regulates immune function will be characterized.